The present invention relates to a magnetic disk apparatus having a plurality of sector periods and, more particularly, to a magnetic disk apparatus comprising a means for changing a sector period.
In a conventional magnetic disk apparatus having a plurality of sector periods, only one counter is provided to define a sector period. When the generation period or timing of a sector pulse is to be changed in order to switch the sector period, the magnetic disk apparatus can only reset the counter or change the count of the counter, and no other means is provided for this purpose. In this conventional magnetic disk apparatus, the timings for sampling the servo surface and data surface of the disk medium are always predetermined.
In the conventional magnetic disk apparatus, since the change of the sector period counter is asynchronous with the counter operation, a short-period sector pulse or a glitch is generated upon a change in the count of the counter. If this short-period sector pulse or glitch is left, it causes a malfunction of a counter that counts the number of sectors per track or a microprocessor (MPU) that controls the write/read operation of the data by using a sector pulse. The A/D conversion start timing of the servo information on the servo surface is not related to the sector period. Thus, when the sector period is changed, the new sector period overlaps the A/D conversion start timing of the servo information on the servo surface which is subjected to sampling, e.g., every sector period, so that sampling is missed. Then, positioning control is degraded.